1. Field of the Invention
The present invention relates to a technique of measuring physical quantities related to a relative movement between two objects, using radio waves. More particularly, it relates to an apparatus for measuring a speed of one object relative to the other, and a direction angle thereof.
2. Description of the Related Art
As an example of the prior art by which the measurement of a speed or direction angle of an object is carried out using radio waves, a system using a frequency-modulated continuous wave (FM-CW system) is known.
For example, using a speed measuring apparatus provided in a moving object such as a vehicle, a flying object, or the like, the measurement of the speed of the moving object is carried out as follows. First, the apparatus transmits a frequency-modulated continuous wave (FM-CW) signal as a radio wave from an antenna toward the ground, and mixes a signal reflected from the ground (i.e., receiving signal) with the transmission signal. The apparatus then extracts from the mixed signal a signal containing a Doppler frequency component based on a speed relative to the ground, and detects the Doppler frequency component to thereby measure the speed of the moving object.
The above FM-CW system, however, poses a problem in that, where the reflection surface for propagation of the transmitted radio wave lies in the close neighborhood of the transmission point, the measuring apparatus is subject to an influence by the reflection in the close neighborhood, and/or an influence by interference of the transmission signal or an interference signal in the receiving system or channel.
Also, where the reflection surface for propagation of the transmitted radio wave lies far away from the transmission point, an influence by the spread of the radio wave beam radiated from the antenna is averaged, and thus the center of power in the frequency band of the receiving signal can be unifiedly determined. However, where the reflection surface for propagation lies in the close neighborhood, a problem occurs in that it is impossible to unifiedly determine the center of power in the frequency band of the receiving signal since conditions of the reflection surface (e.g., the ground) cannot be ignored.
On the other hand, where the direction of a target object is measured using a single antenna, it is necessary to sharpen the beam pattern of the antenna for the purpose of improving the precision of the measurement. To this end, measures must be taken to heighten the frequency of a radio wave signal to be radiated, or to make the size of the antenna per se large. However, the measures are not always effective, much less in the case where the target object lies in the close neighborhood.
To cope with this, an example of the prior art uses two antennas having antenna patterns with slightly different angles. Namely, the example utilizes the fact that the point at which the two antenna patterns cross each other (hereinafter referred to as a "cross point") is unifiedly one point, and thus specifies the direction of the target object.
However, because of complexity of circuit techniques, it has been the state of the art to alternately switch the two antenna patterns. Accordingly, the prior art poses a problem in that, due to a time lag caused by the switching, an error is introduced in the precision of the direction measurement.